Battery systems comprise rechargeable electrical energy stores which are widely used in portable consumer devices and other applications, for example in partially or exclusively electrically operated vehicles. In automotive applications, battery systems and, in particular, lithium-ion battery systems are considered to be a key technology for electrifying the drive system of vehicles. In this case, battery systems and, in particular, lithium-ion battery systems can be of modular construction, depending on the specified use, and usually comprise a plurality of battery cells which are electrically interconnected in series or in parallel.
An important aspect for this technology to become successfully established is, amongst others, the operational reliability of the battery systems, especially of the lithium-ion battery systems which are often operated with large capacitances. In this case, it is necessary to operate the battery system within an optimum temperature range. This is because the battery temperature has a great deal of influence on the provision of power, aging, service life and operational reliability of the battery system. In order to ensure this, battery systems usually have a sophisticated thermal management concept.
During operation of battery systems, the temperature in the battery may increase in such a way that said battery can no longer be cooled by external cooling systems. The main causes of temperature profiles of this kind are internal short-circuits in the battery, for example caused by physical deformations of the battery or by contamination during production of the battery (for example damage to the separator by particles). An internal short-circuit can set in motion further irreversible electrochemical processes in the battery in which heat is released (exothermic reactions). Finally, so-called thermal runaway, a kind of self-accelerated overheating, can result, and this can lead to the overpressure safety valve bursting and toxic gases being expelled or even to the battery cells exploding. Another possible cause of irreversible temperature increases in a battery is overcharging of the battery.
Battery systems usually have safety devices which serve to prevent thermal runaway of the battery system. The objective of this is to identify the risk of irreversible overheating in good time and possibly to rapidly disconnect the battery from the rest of the power supply system. By way of example, safety devices which suppress a current through the battery cell at an elevated internal pressure, which is produced in the case of an elevated temperature in the battery cell, so-called CIDs (Current Interactive Devices), are integrated in battery cells with a relatively large storage capacity, typically of more than 3 ampere hours. Said CIDs provide protection in the case of overloading due to excessively high charging or discharging currents and overloading due to an external short-circuit, these causing thermal runaway, but not in the case of other causes, in particular in the case of an internal short-circuit which is caused by mechanical deformation, particles or dendrites. A further serious disadvantage of safety devices of this kind is that, when they are triggered, the electronics used for monitoring the individual battery cells or for determining the state of charge can be damaged and the entire system can enter an undefined state. A cause of damage to the electronics when a current interruptive device is triggered is that the associated voltage detection means or voltage monitoring means is acted on by high negative voltages which, in a battery which is used in vehicles, can easily amount to several hundred volts. The electronic components used can be protected against such loading only with a considerable amount of expenditure. Therefore, measures of this kind are generally dispensed with for reasons of economy.
In lithium-ion battery cells, novel safety devices are integrated instead, said safety devices internally short-circuiting when they are triggered, this overcoming the abovementioned disadvantages. At the same time, a battery state identification means has to be provided, said battery state identification means identifying triggering of these novel safety devices so that the battery system can be moved to a safe state.